Harmony - Troop Symmetry

Will the ability of upgraded zombies, combined with immunity to slow, balance out the charge attack? Perhaps it would be better to forget about the charge attack and give one a curse, another a weakening strike, and all immunity to slow, for example.

You can't simply equalize units based on abilities; the main difference should be determined by stats (in my opinion), and they should also be used to balance things out. Abilities are just a flavor addition to stats, like clothing; at least most abilities that don't make sense in isolation from stats.